MM-44805: Avoid pointers in JSON parsing in createCategoryForTeamForUser (#20429)

Passing null is a valid JSON input for a pointer struct. So we avoid
passing pointer to pointer.

https://mattermost.atlassian.net/browse/MM-44805

```release-note
NONE
```

api4/channel_category_test.go

@@ -5,6 +5,7 @@ package api4
 
 import (
 	"encoding/json"
+	"fmt"
 	"testing"
 	"time"
 
@@ -90,6 +91,17 @@ func TestCreateCategoryForTeamForUser(t *testing.T) {
 		require.Equal(t, int64(10), customCategory.SortOrder)
 	})
 
+	t.Run("should not crash with null input", func(t *testing.T) {
+		require.NotPanics(t, func() {
+			user, client := setupUserForSubtest(t, th)
+			payload := []byte(`null`)
+			route := fmt.Sprintf("/users/%s/teams/%s/channels/categories", user.Id, th.BasicTeam.Id)
+			r, err := client.DoAPIPostBytes(route, payload)
+			require.Error(t, err)
+			closeBody(r)
+		})
+	})
+
 	t.Run("should publish expected WS payload", func(t *testing.T) {
 		t.Skip("MM-42652")
 		userWSClient, err := th.CreateWebSocketClient()